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Alarming vibration in flight

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glenadavis

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I only have 6 hours on my 2013 RV-12. It has 1500 hours TT. Builder is deceased. I get an intermittent vibration easily felt in the stick and airframe at various speeds and profiles. 60 percent of the time it’s not present. One common thread is I can stop the vibration by applying light back pressure on the stick. Makes me think it’s an elevator issue. I cannot pin it down to airspeed or RPM. Prop was balanced. Can I rule out carb synchronization as back pressure makes it stop? Where should I start looking? Maybe cable tension? Tks.
 
Does this vibration still occur when the autopilot is OFF? With the autopilot ON you will typically feel some very slight pulsing (vibration?) through the control stick as the autopilot servos do their thing.
 
This may be a long shot, but insure that the wing root/fuselage gap is sealed. There have been instances, I know personally of one, where the gap was not sealed and affected the stabilator resulting in large vibration in flight.
The Production Acceptance Procedure was recently changed to include this notation:
  • Ensure gap between wing root and fuselage is sealed. Refer to KAI Section 38iS/U.
 
georgemohr said:
A note of caution: if you are experiencing some form of control flutter, this is super serious and can turn catastrophic quickly. I would not fly until you have reasonable certainty that you know the cause.
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Absolutely, all the rest of the diagnosis should be on the ground. If it’s elevator related should take little more than having the stick held while someone moves the control surface outside. Get a copy of the drawings from VANs and start checking the linkages. Check VANs SB’s as well as there appears to be a recent one regarding the pushrod assembly.
 
If its anything like the -14, the fwd elevator fairings can sit just "proud" of the horz stab at times during level flight causing a buffetting type feeling. This can be verified by looking over your shoulder when it happens and you can see the elevator move slightly in conjunction with the control stick
 
I would download Van's Production Acceptance Procedures and verify all of the measurements (e.g. stabilator & AST deflections). Include checking the elevator cable tension, then check for play at every moving joint from the stick back to the anti-servo tab (including the AST trim linkage). Include the integrity of the stabilator balance arm & weights.

Comments about the potential seriousness of this flutter/vibration are well put.
 
DHeal said:
Does this vibration still occur when the autopilot is OFF? With the autopilot ON you will typically feel some very slight pulsing (vibration?) through the control stick as the autopilot servos do their thing.
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Autopilot was off.
 
glenadavis said:
I only have 6 hours on my 2013 RV-12. It has 1500 hours TT. Builder is deceased. I get an intermittent vibration easily felt in the stick and airframe at various speeds and profiles. 60 percent of the time it’s not present. One common thread is I can stop the vibration by applying light back pressure on the stick. Makes me think it’s an elevator issue. I cannot pin it down to airspeed or RPM. Prop was balanced. Can I rule out carb synchronization as back pressure makes it stop? Where should I start looking? Maybe cable tension? Tks.
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I have seen a problem like this. It might be the valve covers vibrating against the cowl scoop. The air scoop in the bottom cowl is very close to the valve covers. Take the top cowl off. Stand on the left side of the engine and pull on the engine in an area as close to the prop as possible. At the same time take a piece of paper and slowly pull in past the valve covers. If you feel any drag on the paper while pulling on the engine, it is probably the source of the noise. It may take two people to preform this simple test.

If the paper drags check the following: The engine mounts allow the engine to move within the cowl. A SB was issued regarding soft or incorrect engine mounts supplied with the kit. This could contribute to this problem. There is a SB on the four engine mount screws. They should also be checked to see if they are the correct torque and none are missing or lose. If this checks out, a little fiberglass work on the air scoop will be necessary to give needed clearance for the valve covers.
 
I had our anti servo tab on the stabilator begin to flutter at certain cg loadings. Check for slop in the anti servo tab by lifting up on the trailing edge. There should be minimal play in the assembly.

It was easily repaired using close tolerance oversized hardware and installing doublers on the trim arm. I also installed bushings in the linkage arm.
 
glenadavis said:
I only have 6 hours on my 2013 RV-12. It has 1500 hours TT. Builder is deceased. I get an intermittent vibration easily felt in the stick and airframe at various speeds and profiles. 60 percent of the time it’s not present. One common thread is I can stop the vibration by applying light back pressure on the stick. Makes me think it’s an elevator issue. I cannot pin it down to airspeed or RPM. Prop was balanced. Can I rule out carb synchronization as back pressure makes it stop? Where should I start looking? Maybe cable tension? Tks.
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I had the exact problem on my RV10 and it did concern me. It only seemed to happen in rough air. I did find that the elevator hinges had some play when I shook the elevator. I removed the elevator and I ordered new parts from Vans and replaced them. There is no longer any slop in the elevator and yes the problem resolved. I do agree this is a problem that should not be ignored. Check for elevator play on your next preflight
 
glenadavis said:
I only have 6 hours on my 2013 RV-12. It has 1500 hours TT. Builder is deceased. I get an intermittent vibration easily felt in the stick and airframe at various speeds and profiles. 60 percent of the time it’s not present. One common thread is I can stop the vibration by applying light back pressure on the stick. Makes me think it’s an elevator issue. I cannot pin it down to airspeed or RPM. Prop was balanced. Can I rule out carb synchronization as back pressure makes it stop? Where should I start looking? Maybe cable tension? Tks.
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If you give it just a hair more power, does it stop? (until the airplane speeds up a knot or two). If this is the case, then I would suspect worn dog gear in the gearbox. Its usually only present through a wide RPM range of (iirc) 4800-5200. Pulling back on the stick would make the vibration go away as the prop will have more load on it.
 
rcarsey said:
If you give it just a hair more power, does it stop? (until the airplane speeds up a knot or two). If this is the case, then I would suspect worn dog gear in the gearbox. Its usually only present through a wide RPM range of (iirc) 4800-5200. Pulling back on the stick would make the vibration go away as the prop will have more load on it.
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We just went thru this on a friend's RV-12 w/ 500TT. How many hours on engine gearbox - 1500TT? Has it ever been rebuilt? My guess is worn splines / clutch dogs in the gearbox.
 
For aircraft with cable driven controls, improper cable tension can create a vibration in a flight control, and in fact can change the characteristics of the control system allowing flutter to develop. The first thing is to check rigging and cable tension before further flight.
 
RVDan said:
For aircraft with cable driven controls, improper cable tension can create a vibration in a flight control, and in fact can change the characteristics of the control system allowing flutter to develop. The first thing is to check rigging and cable tension before further flight.
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I agree that the OP’s problem description points to a need for a gearbox inspection.
 
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CTCole said:
I would download Van's Production Acceptance Procedures and verify all of the measurements (e.g. stabilator & AST deflections). Include checking the elevator cable tension, then check for play at every moving joint from the stick back to the anti-servo tab (including the AST trim linkage). Include the integrity of the stabilator balance arm & weights.

Comments about the potential seriousness of this flutter/vibration are well put.
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Not that familiar with the RV controls, but if you suspect the vibration is from one of the controls, then also check that the surface not only has the weights installed, but that the control surface itself is balanced correctly. Paint added to a control surface can unbalance a surface.
 
Per my post #13 above…. My friend’s RV-12 (500TT) ran smooth at takeoff and cruise but had very bad sudden vibration as soon as throttle was pulled back even a few hundred rpm. Happened repeatedly and took a lot of convincing that gearbox was the cause. We sent gearbox for inspection and so many parts needed replacement it was cheaper to buy whole new gearbox. Problem completely solved…
 
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